Battery protective circuit with current-amount control circuit

ABSTRACT

A battery protective circuit which can ensure the safety and reliability of a rechargeable secondary battery is provided. Personal digital assistants include a main circuit ( 30 ) and a battery block ( 60 ). The battery block ( 60 ) includes a battery ( 20 ) and a current-amount control circuit ( 50 ). The battery ( 20 ) is charged via an AC adapter. The current-amount control circuit ( 50 ) includes a current and temperature detecting circuit (for example a PTC element) operative to reduce a current amount when an amount of current flowing in the battery ( 20 ) approaches a boundary value of a charge-guaranteed region in which the battery ( 20 ) is rechargeable.

TECHNICAL FIELD

The present invention relates to a battery protective circuit, and moreparticularly to a configuration for preventing a value of currentflowing in a battery from exceeding a guaranteed current when a short orovercharge occurs.

BACKGROUND ART

Recently, personal digital assistants such as a portable telephone, anotebook-sized personal computer and a video camera are widely used.These personal digital assistants use a battery for supplying power. Arechargeable secondary battery is used for such a battery.

When current flowing in a battery increases for some reason (a short ofan electric circuit or charge at an overvoltage and the like), thebattery may generate excessive heat and possibly become degraded ordamaged.

Therefore, these tools are conventionally equipped with a batteryprotective circuit for protecting the battery. An example of the batteryprotective circuit includes a PTC (Positive Temperature Coefficient)element and a thermal protector. The PTC element or the thermalprotector serves as a current and temperature detecting circuit,operating in such a manner that electric resistance thereof increases asa larger current flows in the element and temperature becomes higher,and electric resistance thereof increases rapidly to suppress currentwhen a certain temperature is reached. Further, a thermistor has itsresistance value changed as an ambient temperature rises.

The conventional battery protective circuit, however, has a problem asdescribed below. Referring to FIG. 12, the problem of the conventionalbattery protective circuit will be described.

With respect to FIG. 12, the ordinate and the abscissa respectivelyrepresent voltage and current. A represents a charge-guaranteed regionin which a battery is rechargeable, and BZ represents a protectionregion in which a current and temperature detecting circuit such as aPTC element or a thermal protector is functional.

The charge-guaranteed region A represents a relation between currentflowing in the battery and voltage across terminals of the battery. Theprotection region BZ represents a relation between current flowing inthe PTC element or the like (and the battery) and voltage acrossterminals of the PTC element or the like.

The charge-guaranteed region A is a region in which the battery canprotect itself, and the protection region BZ is a region in which thecurrent and temperature detecting circuit is functional.

When the value of current flowing in the battery enters the protectionregion BZ for some reason, the internal resistance of the PTC element orthe like increases. As a result, an amount of current flowing in thecircuit decreases.

The conventional battery protective circuit has performed its protectingfunction when heavy current flows, regardless of the charge-guaranteedregion A, as shown in FIG. 12.

Therefore, for the current value between the charge-guaranteed region Aand the protection region BZ, any safety and reliability of the batteryis not assured. Thus, unfortunately, for some types of batteries, thebattery cannot protect itself, and in addition, the safety andreliability of a device operated by the battery cannot be assured.

Then, the present invention is made to solve the above mentionedproblem, and its object is to provide a battery protective circuit whichcan ensure the safety and reliability of a rechargeable battery and adevice operated by the battery.

DISCLOSURE OF THE INVENTION

According to an aspect, the present invention provides a batteryprotective circuit for a rechargeable battery, including acurrent-amount control circuit including a current and temperaturedetecting circuit provided near the battery, operative to detect a valueof current flowing in the battery and an ambient temperature, and todecrease the current value when the current value and the ambienttemperature reach a value of a protection region, wherein the minimumcurrent value in the protection region is less than the maximum currentvalue in the charge-guaranteed region in which the battery isrechargeable, and the maximum current value in the protection region isgreater than the maximum current value in the charge-guaranteed region.

According to another aspect, the present invention provides a batteryprotective circuit for a rechargeable battery, including: acurrent-amount control circuit including a current and temperaturedetecting circuit provided near the battery, operative to detect a valueof current flowing in the battery and an ambient temperature, and todecrease the current value when the current value and the ambienttemperature reach a value of a first protection region; and aninterconnection layer supplying current to be flown in the battery,including a meltable portion to be melted and cut off when the value ofcurrent flowing in the battery reaches a value of a second protectionregion. The minimum current value in the first protection region is lessthan the maximum current value in the charge-guaranteed region in whichthe battery is rechargeable, and the maximum current value in the firstprotection region is greater than the maximum current value in thecharge-guaranteed region. The minimum current value in the secondprotection region is less than the maximum current value in the firstprotection region, and the minimum current value in the secondprotection region is greater than the maximum current value in thecharge-guaranteed region. In the interconnection layer, the meltableportion has a relatively small cross sectional area, while a portionother than the meltable portion of the interconnection layer has arelatively large cross sectional area.

Preferably, at least two or more meltable portions of theinterconnection layer are arranged.

The aforementioned battery protective circuit can decrease the currentvalue before degradation and damage of the battery, even when the valueof current flowing into the battery increases.

As a result, the battery can surely be protected, and the safety andreliability of the battery and the device operated by the battery can beimproved.

Further, when the current amount approaches a boundary region of anoperating condition of the current and temperature detecting circuit dueto overcharge, charge in reverse direction or the like, theinterconnection layer is melted and cut off at the time when thecurrent-amount control circuit is not yet damaged, and therefore thecurrent is interrupted.

Therefore, the undesirably high temperature of the battery can beprevented and the current-amount control circuit may not be burdened. Asa result, the overall reliability and safety of the device including thebattery and the current-amount control circuit can be improved.

According to a further aspect, the present invention provides a batteryprotective circuit for a rechargeable battery, including: acurrent-amount control circuit including a current and temperaturedetecting circuit provided near the battery, operative to detect a valueof current flowing in the battery and an ambient temperature, and todecrease the current value when the current value and the ambienttemperature reach a value of a first protection region; and aninterconnection layer supplying current to be flown in the battery,including a meltable portion to be melted and cut off when the value ofcurrent flowing in the battery reaches a value of a second protectionregion. The minimum current value in the first protection region is lessthan the maximum current value in the charge-guaranteed region in whichthe battery is rechargeable, and the maximum current value in the firstprotection region is greater than the maximum current value in thecharge-guaranteed region. The minimum current value in the secondprotection region is less than the maximum current value in the firstprotection region, and the minimum current value in the secondprotection region is greater than the maximum current value in thecharge-guaranteed region. In the interconnection layer, the meltableportion has a relatively small cross sectional area, while a portionother than the meltable portion of the interconnection layer has arelatively large cross sectional area. In the interconnection layer, thegreater the current value in the second protection, the shorter the timefor the meltable portion be melted and cut off.

The aforementioned battery protective circuit can decrease the currentvalue before degradation and damage of the battery, even when the valueof current flowing into the battery increases, so that the battery cansurely be protected, and in addition, the safety and reliability of thebattery and the device operated by the battery can be improved.Furthermore, when the current amount approaches a boundary region of anoperating condition of the current and temperature detecting circuit dueto overcharge, charge in reverse direction or the like, theinterconnection layer is melted and cut off at the time when thecurrent-amount control circuit is not yet damaged, and the current isinterrupted. Therefore, the undesirably high temperature of the batterycan be prevented and the current-amount control circuit may not beburdened. As a result, the overall reliability and safety of the deviceincluding the battery and the current-amount control circuit can beimproved. Further, as the current value in the second protection regionbecomes greater, the time for the meltable portion to be melted and cutoff becomes shorter. Therefore, even when heavy current flows, themeltable portion is not melted and cut off, if the time of current flowis short enough. As a result, even when the terminal of the batterycauses a momentary short-circuit, the meltable portion does not melt, ifthe moment is short enough. Therefore, a short circuit over such a shorttime that does not affect the safety may not result in a failure of thebattery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of a main portion of aportable telephone in accordance with a first embodiment.

FIG. 2 is a graph illustrating a battery protective function by acurrent-amount control circuit 50 in accordance with the firstembodiment.

FIG. 3 is a diagram showing an exemplary configuration of the mainportion of the portable telephone in accordance with the firstembodiment.

FIG. 4 is a diagram illustrating a structure of a main portion of aportable telephone 1 in accordance with a second embodiment.

FIG. 5A is a cross sectional view illustrating a structure of aninterconnection layer 24 in accordance with the second embodiment, takenalong line A—A in FIG. 4.

FIG. 5B is a cross sectional view illustrating the structure ofinterconnection layer 24 in accordance with the second embodiment, takenalong line B—B in FIG. 4.

FIG. 5C is a cross sectional view illustrating the structure ofinterconnection layer 24 in accordance with the second embodiment, takenalong line C—C in FIG. 4.

FIG. 6 is a graph illustrating a battery protective function by acurrent-amount control circuit 50 and interconnection layer 24 inaccordance with the second embodiment.

FIG. 7 is a diagram showing an exemplary configuration of the mainportion of portable telephone 1 in accordance with the secondembodiment.

FIG. 8 is a diagram showing an exemplary configuration of the mainportion of portable telephone 1 in accordance with the secondembodiment.

FIG. 9 is a diagram showing an exemplary configuration of the mainportion of portable telephone 1 in accordance with the secondembodiment.

FIG. 10 is a graph showing a relation between the value of current andthe time at which each element functions, in a battery protectivecircuit in accordance with the second embodiment of the presentinvention.

FIG. 11 is a graph showing a specific relation between the value ofcurrent and the time at which each element functions, in a batteryprotective circuit in accordance with the second embodiment of thepresent invention.

FIG. 12 is a graph illustrating a problem in a conventional batteryprotective circuit.

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described with reference tothe figures. Note that the same or corresponding parts in the figuresare denoted with the same reference characters and the descriptionthereof is not repeated.

First Embodiment

Referring to FIG. 1, a configuration for battery protection inaccordance with a first embodiment will be described. It is noted thatin the following description, a portable telephone is taken as anexample of a personal digital assistant for the purpose of illustration.FIG. 1 shows a configuration of a main portion of a portable telephonein accordance with the first embodiment.

Turning now to FIG. 1, the portable telephone includes a main circuit 30including a processing circuit and the like for sending and receiving asignal, a battery block 60 for supplying electricity to main circuit 30,and a control circuit 80.

Battery block 60 includes a battery 20 and a current-amount controlcircuit 50 for protecting battery 20. When battery 20 is charged, an ACadapter 70 is connected to an AC adapter terminal 72. AC adapterterminal 72 is electrically connected to a node P, from which currentflows into battery 20.

Battery block 60 is contained inside a housing for the portabletelephone. Alternatively, it may be formed removable from the housing ofthe portable telephone.

Current-amount control circuit 50 is configured, for example, with a PTCelement which is a current and temperature detecting circuit, athermistor or the like which is a temperature sensing element, or acomposite element thereof. The current and temperature detecting circuitas used herein refers to a PTC element, a thermal protector or the like,and it has a function of continuously detecting a current value and atemperature to control the value of current. It is noted that current isinterrupted by the current and temperature detecting circuit when acertain value of current and temperature are reached. In FIG. 1,current-amount control circuit 50 is configured with PTC element 55. PTCelement 55 is connected to the side of the negative terminal of battery20.

Main circuit 30 is connected with node P. Main circuit 30 is configuredwith an electronic component including an interconnection, a resistance,a capacitor, a coil and the like, and it is operated by power supplyfrom battery 20. Main circuit 30 can monitor a change in resistancevalue (signal S) of current-amount control circuit 50. For example, maincircuit 30 can be configured to include a circuit for controlling thecharge such that the voltage at node P can be kept constant by signal S.

Furthermore, main circuit 30 includes a circuit (such as a clock)operated by supply voltage received from node R, which is electricallyconnected to AC adapter terminal 72.

Control circuit 80 controls voltage and current which is supplied fromAC adapter 70 to battery 20 and main circuit 30. Control circuit 80includes a resistance element RE and a transistor T. Transistor T isconnected between node R and one terminal of resistance element RE, theother terminal of which is connected to node P. Transistor T turns on inresponse to a control signal received from main circuit 30. The currentand voltage for charging the battery is controlled under the control ofcontrol circuit 80.

PTC element 55 forming current-amount control circuit 50 graduallyincreases in electric resistance as the value of current flowing in theelement increases and the ambient temperature increases. Then, when thecurrent value and the ambient temperature exceed prescribed values, theresistance rapidly increases. Therefore, PTC element can operate todecrease the value of current flowing in battery 20 in response to thevalue of current flowing in battery 20 and the ambient temperature.

Referring now to FIG. 2, the battery-protecting function bycurrent-amount control circuit 50 in accordance with the firstembodiment will be described. In FIG. 2, the ordinate and the abscissarespectively represent voltage and current, A represents acharge-guaranteed region in which the battery is rechargeable withoutdamage, and B represents the protection region in which the current andtemperature detecting circuit (PTC element 55) in accordance with thefirst embodiment is functional.

The charge-guaranteed region A is a region in which the battery isrechargeable without damage, that is, the battery can protect itself.Charge-guaranteed region A ranges from 0 ampere to F0 ampere. In thefigure, F0 is about (2+α) amperes (the current value varies with batteryrating).

The protection region B of the current and temperature detecting circuitis a region where the battery is protected. Protection region B rangesfrom the current value F1 to the current value F2. The current value F1is included in a boundary region of charge-guaranteed region A. In thefigure, F1 is about 2 amperes, and F2 is about (12+β) amperes.

As the value of current flowing in battery 20 enters protection regionB, the current and temperature detecting circuit operates to increaseits resistance and decrease the current flowing into battery 20. At thispoint, the current and temperature detecting circuit performs itsprotecting function from the vicinity of boundary value ofcharge-guaranteed region A of battery 20. Therefore, when the value ofcurrent flowing in battery 20 gets close to the boundary value ofcharge-guaranteed region A, the value of current flowing into battery 20can be shifted to charge-guaranteed region A.

As a result, degradation and damage of battery 20 can surely beprevented, and the overall safety and reliability of the deviceincluding the battery can be assured.

Note that the configuration of current-amount control circuit 50 is notlimited to the one shown in FIG. 1. As an example of current-amountcontrol circuit 50, a thermal protector may be used. Further, as shownin FIG. 3, battery 20 may have its positive terminal connected to PTCelement 55 and its negative terminal connected to a temperature sensingelement 56, such as a thermistor.

Second Embodiment

A configuration for battery protection in accordance with a secondembodiment will be now described. According to the second embodiment, aninterconnection layer having a meltable portion to be melted and cut offin accordance with a current value is arranged in addition to the abovementioned current-amount control circuit 50, for battery 20.

FIG. 4 illustrates a structure of a main portion of a portable telephonein accordance with the second embodiment of the present invention.Referring to FIG. 4, a portable telephone 1 includes a housing 2, aprinted board 10 as an insulating substrate, a main circuit 30, abattery 20 and an antenna 40. A large part of antenna 40 is accommodatedin housing 2, and can be extended to protrude from housing 2 whenportable telephone 1 is used.

Printed board 10 is fixed to housing 2. Main circuit 30 is provided onprinted board 10. Main circuit 30 is supplied with electricity frombattery 20.

Battery 20 is fixed to printed board 20. Battery 20 has a battery core21 as a power generation element, an exterior member 26, a positiveterminal 22 and a negative terminal 23. Positive terminal 22 andnegative terminal 23 are electrically connected to battery core 21.

An interconnection layer 24 and PTC element 55 as a current-amountcontrol circuit are also arranged on printed board 10. PTC element 55and interconnection layer 24 are connected between positive terminal 22and main circuit 30. An interconnection layer 31 is connected betweennegative terminal 23 and the main circuit.

Interconnection layer 24 is formed from copper. An interconnectionportion 24 b at both ends of interconnection layer 24 is formed to berelatively wide, and meltable portion 24 a located at the center portionis formed to be relatively narrow.

FIG. 5A shows a cross section seen along line A—A in FIG. 4, and FIG. 5Bshows a cross section seen along line B—B in FIG. 4. FIG. 5C shows across section seen along line C—C in FIG. 4.

Referring to FIG. 5A, meltable portion 24 a is formed on printed board10. The cross section of meltable portion 24 a is generally rectangular.The height of meltable portion 24 a is T1, and the width thereof is W1.Referring to FIG. 5B, interconnection portion 24 b is formed on printedboard 10. The cross section of interconnection portion 24 b is generallyrectangular. The height of interconnection portion 24 b is T1 and thewidth thereof is W2 which is greater than W1. Referring to FIG. 5C, thelength of meltable portion 24 a is L.

The cross sectional area of meltable portion 24 a is smaller than thecross sectional area of interconnection portion 24 b. Therefore, wheninterconnection layer is supplied with current, the density of currentpassing through interconnection portion 24 b is relatively small, andthe density of current passing through meltable portion 24 a is greater.Accordingly, meltable portion 24 a rapidly generates heat when currentexceeds a prescribed value. This heat generation melts meltable portion24 a. Interconnection layer 24 is thereby broken.

Smaller cross sectional area (T1×W1) of meltable portion 24 a increasesthe resistance of meltable portion 24 a, so that meltable portion 24 acan be melted and cut off with a small current. Alternatively, longerlength L of meltable portion 24 a increases the resistance of meltableportion 24 a, so that meltable portion 24 a can be melted and cut offwith a small current. Therefore, the value of current at which meltableportion 24 a is melted and cut off can be set by adjusting the lengthand cross sectional area of meltable portion 24 a. For example, whenheight T1, width W1 and length L are respectively set to about 35 μm,about 150 μm and 10 mm, meltable portion 24 a is melted and cut off atabout 7 amperes. When two such interconnection layers are disposed inparallel, meltable portion 24 a is melted and cut off at about 14amperes.

Referring now to FIG. 6, the battery-protecting function byinterconnection layer 24 and current-amount control circuit 50 will bedescribed. In FIG. 6, the ordinate and the abscissa respectivelyrepresent voltage and current. A represents a charge-guaranteed regionin which the battery is rechargeable without damage, and B represents aprotection region in which the current and temperature detecting circuit(for example a PTC element, a thermal protector and the like) formingcurrent-amount control circuit 50 can function. Further, C represents aprotection region of the interconnection layer.

Charge-guaranteed region A in which the battery is rechargeable withoutdamage ranges from 0 ampere to F0 ampere. In the figure, F0 is about(2+α) amperes.

Protection region B of the current and temperature detecting circuitranges from the current value F1 to the current value F2, and thecurrent value F1 is included in the boundary region of charge-guaranteedregion A. In the figure, F1 is about 2 amperes, and F2 is about (12+β)amperes.

In this case, interconnection layer 24 is formed such that meltableportion 24 a is melted when the value of current flowing in the batterybecomes equal to or more than F3. Here, the current value F3 is set to avalue included in the boundary region of the protection region of thecurrent and temperature detecting circuit. In the figure, F3 is about 12amperes.

In other words, in the second embodiment, interconnection layer 24 isformed such that the protection region by interconnection layer 24 (thecurrent region in which meltable portion 24 a is melted and theinterconnection layer is cut off) overlaps the boundary value of theprotection region of the current and temperature detecting circuit.

If the current flowing in the battery has a current value withincharge-guaranteed region A (0 ampere to F0 ampere), the battery can becharged without damage.

When the value of current flowing in the battery enters protectionregion B, the current and temperature detecting circuit operates toincrease its resistance and decrease the current flowing into thebattery. At this point, current-amount control circuit 50 performs itsprotecting function from the vicinity of the boundary value ofcharge-guaranteed region A of the battery.

Further, when the value of current flowing in the battery gets close tothe boundary value of protection region B, meltable portion 24 a ismelted, and interconnection layer 24 is cut off, so that the currentflowing in current-amount control circuit 50 and battery 20 isinterrupted. Therefore, a short between PTC elements due tocarbonization of PTC element caused by the current exceeding protectionregion B can be prevented. Furthermore, the current-limiting effect byPTC element 55 has a time delay, of which effect on battery 20 can alsobe prevented.

It is noted that in the portion where two regions overlap, for example,in the region where charge-guaranteed region A and protection region Bof current and temperature detecting circuit overlap (that portion inwhich the current value is not less than F1 and not more than F0),charge-guaranteed region A of the battery, which is at the left side ofthese regions, is designed to function with priority. Further, in theregion where protection region B of the current and temperaturedetecting circuit and protection region C of the interconnection layeroverlap (that portion in which the current value is not less than F3 andnot more than F2), protection region B of the current and temperaturedetecting circuit, which is at the left side of these regions, isdesigned to function with priority.

In this manner, according to the second embodiment, when a short or thelike causes heavy current to flow in battery core 21, PTC element 55 canoperate to decrease the value of current flowing in battery core 21. Inaddition, when it comes close to such an environment that is out of thecondition ensuring normal operation of PTC element 55 due to charge atan overvoltage, charge in reverse direction or the like, meltableportion 24 a is melted and cut off.

Since this can interrupt the current, the undesirably high temperatureof battery core 21 can be prevented. In addition, degradation and damageof current-amount control circuit 50 can be prevented. As a result, theoverall safety and reliability of the device including the battery canbe assured.

An example of such a relation between interconnection layer 24 and thecurrent-amount control circuit is as shown in FIGS. 7 to 9. In anexample shown in FIG. 7, current-amount control circuit 50 is configuredwith PTC element 55 and thermistor 56. Interconnection layer 24 and PTCelement 55 are connected between the positive terminal of battery 20 andmain circuit 30, and thermistor 56 is connected with the side of thenegative terminal of battery 20.

In an example shown in FIG. 8, interconnection layer 24 is arrangedbetween main circuit 30 and the positive terminal of battery 20, and PTCelement and thermistor 26 configuring current-amount control circuit 50is connected with the side of the negative terminal of battery 20.

Further, in an example shown in FIG. 9, PTC element 55 and thermistor 26configuring current-amount control circuit 50 is connected with the sidewith lower potential. Interconnection layer 24 is connected between PTCelement 55 and a ground terminal.

FIGS. 7 to 9 show PTC element 55 and thermistor 56 configuringcurrent-amount control circuit 50, but this invention is not limitedthereto.

FIG. 10 is a graph showing the relation between the value of current andthe time at which each element may function, in the battery protectivecircuit in accordance with the second embodiment of the presentinvention. In FIG. 10, the ordinate shows the time required for eachelement to start an operation, and the abscissa shows the current value.Curve 110 shows the relation between the time and the value of currentat which the current and temperature detecting circuit (such as a PTCelement, a thermal protector or the like) configuring current-amountcontrol circuit 50 may operate. Curve 120 shows the relation between thetime and the value of current necessary for meltable portion 24 a ofinterconnection layer 24 to be melted and cut off. Referring to FIG. 10,as shown with curve 110, as the value of current flowing in thetemperature detecting circuit (such as a PTC element, a thermalprotector or the like) becomes greater, the time required forcurrent-amount control circuit 50 to start an operation becomes shorter.Similarly, as shown with curve 120, as the value of current flowing ininterconnection layer 24 becomes greater, the time required for meltableportion 24 a to start melting and cutting off becomes shorter. Both ofcurve 110 and curve 120 are convex downward, which shows that thecorresponding element takes shorter time to operate as the current valuebecomes greater.

The protection region of the current and temperature detecting circuitranges from the current value F1 to the current value F2, and thecurrent value F1 is included in the boundary region of thecharge-guaranteed region.

In this case, for interconnection layer 24, meltable portion 24 a ismelted when the value of current flowing in the battery reaches equal toor more than F3. Here, the current value F3 is set to be a value whichis included in the boundary region of the protection region of thecurrent and temperature detecting circuit.

Next, three interconnection layers 24 were prepared, with 0.2 mm of thewidth W1, 35 μm of thickness T1, and 10 mm of the length of meltableportion 24 a shown in FIG. 5A. These were connected in parallel, and inthis interconnection layer, the time and the value of current at whichthe meltable portion began melting were measured in a high temperatureatmosphere and a low temperature atmosphere. Further, the time and thevalue of current at which PTC began operating were measured in a hightemperature atmosphere and a low temperature atmosphere. The result isshown in FIG. 11. In FIG. 11, the ordinate shows the time required foreach element to start an operation, and the abscissa shows the currentvalue. Curve 201 shows the relation between the time and the value ofcurrent necessary for PTC to operate in the high temperature atmosphere.Curve 202 shows the relation between the time and the value of currentnecessary for PTC to operate in the low temperature atmosphere. Curve203 shows the relation between the time and the value of currentnecessary for meltable portion 24 a to be melted and cut off in the hightemperature atmosphere. Curve 204 shows the relation between the timeand the value of current necessary for meltable portion 24 a to bemelted and cut off. As can be seen from FIG. 11, in the interconnectionlayer in accordance with this invention, as the value of current atwhich meltable portion 24 a is melted and cut off becomes greater, thetime required for meltable portion 24 a to start melting and cutting offbecomes shorter. In addition, it can be seen that meltable portion 24 ais melted and cut off in a shorter time in the high temperatureatmosphere compared with in the low temperature atmosphere.

For the protective circuit in accordance with the present invention, anexternal short (so called chain-short) with not more than 50 m Ω ofresistance value and about one second of the duration may not melt andcut off meltable portion 24 a, and therefore the battery can be reused.

Although the first and second embodiments have been described above,various modifications may be made on the embodiments described herein.

Although a portable telephone is taken as an example of a personaldigital assistant, the present invention is not limited thereto, and maybe applied to a notebook-sized personal computer, a video tape recorderand the like.

Any of a lithium cell, a nickel-cadmium battery and a polymer batterymay be used as battery 20.

The relation between PTC element 55 and thermistor 56 andinterconnection layer 24 which are arranged for the positive terminal 22and the negative terminal 23 of the battery is not limited to the onedescribed above.

Various kinds of material can be used rather than copper and the like,as a material of interconnection layer 24. More specifically,interconnection portion 24 b may be formed of a material with highermelting point and meltable portion 24 a may be formed of a material withlower melting point.

Further, the number of interconnection layers 24 is not limited to one,but may be changed properly as needed. In case where a plurality ofinterconnection layers 24 are provided, any one of meltable portion 24 ais surely melted, and therefore the reliability of the device is furtherimproved.

Still further, the shape of the meltable portion in the interconnectionlayer is not limited to the linear shape as shown. The meltable portionmay be formed, for example, to extend in a serpentine shape in order tosecure the length.

In accordance with the present invention, even when the value of currentflowing into the battery increases, the value of current can bedecreased before degradation and damage of the battery. As a result, thebattery can surely be protected, and the reliability and safety of thebattery and the device operated by the battery can be improved.

In accordance with the present invention, the current amount approachesthe boundary region of the operating condition of the current andtemperature detecting circuit, due to overcharge, charge in reversedirection or the like, the interconnection layer is melted and cut offat the time when the current-amount control circuit is not yet damaged,and the current is interrupted.

Therefore, the undesirably high temperature of the battery can beprevented, and in addition, the current-amount control circuit may notbe burdened. As a result, the reliability and safety of the battery andthe device operated by the battery can be improved.

In addition, in accordance with the present invention, a plurality ofthe interconnection layers having meltable portions may be provided, sothat current can be interrupted at respective different positions whenit enters the protection region of the interconnection layer. Inparticular, when the identical interconnection layers are provided inparallel, the reliability can be enhanced.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

Industrial Applicability

A battery protective circuit in accordance with the present inventioncan be applied to a portable telephone, a notebook-sized personalcomputer, a word processor, a liquid crystal television, a VTR withcamera and the like.

What is claimed is:
 1. A battery protective circuit comprising acurrent-amount control circuit having a current and temperaturedetecting circuit operative to detect a value of current flowing in arechargeable battery including a battery core as a power generatingelement and an electrode terminal electrically connected to the batterycore, and to decrease said value of current when said value of currentand an ambient temperature of said battery reaches a value in aprotection region, wherein a minimum current value in the protectionregion of said current and temperature detecting circuit is less than amaximum current value in a charge-guaranteed region in which saidbattery is rechargeable without damage and said battery can protectitself, and a maximum current value in said protection region is greaterthan the maximum current value in said charge-guaranteed region.
 2. Abattery protective circuit comprising a current-amount control circuithaving a current and temperature detecting circuit operative to detect avalue of current flowing in a rechargeable battery including a batterycore as a power generating element and an electrode terminalelectrically connected to the battery core, and to decrease said valueof current when said value of current and an ambient temperature of saidbattery reaches a value in a first protection region, further comprisingan interconnection layer including a meltable portion to be melted andcut off when a value of current flowing in said battery reaches a valuein a second protection region, a cross sectional area of said meltableportion being relatively smaller than a cross sectional area excludingsaid meltable portion, wherein a minimum current value in a firstprotection region of said current and temperature detecting circuit isless than a maximum current value in a charge-guaranteed region in whichsaid battery is rechargeable without damage and said battery can protectitself, and a maximum current value in said first protection region isgreater than the maximum current value in said charge-guaranteed region,and a minimum current value in said second protection region is lessthan the maximum current value in said first protection region, and theminimum current value in said second protection region is greater thanthe maximum current value in said charge-guaranteed region.
 3. Thebattery protective circuit according to claim 2, wherein at least twomeltable portions of said interconnection layer are arranged.